Imaging lens

ABSTRACT

An imaging lens includes a first lens having negative refractive power; a second lens having positive refractive power; a third lens having positive refractive power; a stop; and a fourth lens having positive refractive power arranged in the order from an object side to an image plane side. The first lens has an image plane-side surface having a positive curvature radius. The second lens has an image plane-side surface having negative curvature radius. The third lens has an image plane-side surface having a negative curvature radius. The fourth lens has an object-side surface having a positive curvature radius and an image plane-side surface having a negative curvature radius. The first lens has a specific focal length and a specific Abbe&#39;s number to satisfy specific conditional expressions.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to an imaging lens for forming an image ofan object on an imaging element such as a CCD sensor and a CMOS sensor.In particular, the present invention relates to an imaging lens suitablefor mounting in a relatively small camera such as a cellular phone, adigital still camera, a portable information terminal, a securitycamera, a vehicle onboard camera, a network camera, a video conferencingcamera, a fiberscope, and an encapsulated endoscope.

In these years, there have been available some vehicles equipped with aplurality of cameras for a purpose of enhancing convenience andsecurity. For example, in case of a vehicle equipped with a backupcamera to take an image behind the vehicle, since a driver can see therear view of the vehicle on a monitor upon backing up the vehicle, thedriver can safely move the vehicle backward without hitting an objecteven if any, although such an object is not visible from the driver dueto shadow of the vehicle. Such a camera mounted on a vehicle, i.e., aso-called onboard camera, is expected to be continuously on demand.

The onboard cameras are often accommodated in a relatively small spacesuch as in a backdoor, a front grill, a side mirror, and inside of thevehicle. For this reason, an imaging lens to be mounted in the onboardcamera is required to have a compact size. Further, the onboard camerais required to be compatible with a high resolution resulting from ahigh-pixel density imaging element, and to have a wide angle to becompatible with a wide imaging range. However, it is difficult to attaina small size and compatibility with the high resolution whilesatisfactorily correcting aberrations, and further attain a wide imagingangle. For example, when a size of an imaging lens is reduced,refractive power of each lens tends to become stronger. Accordingly, itis difficult to satisfactorily correct aberrations. Therefore, uponactually designing the imaging lens, it is important to satisfy thosedemands in a balanced manner.

As a wide-angle imaging lens that has a wide imaging angle, for example,there is known an imaging lens described in Patent Reference. Theimaging lens includes a first lens that has a shape of a meniscus lensdirecting a convex surface thereof to an object side and is negative; asecond lens that has a shape of a meniscus lens directing a concavesurface thereof to the object side and is positive; a third lens that ispositive; and a fourth lens that is positive, arranged in the order fromthe object side.

Patent Reference: Japanese Patent Application Publication No.2011-145665

According to the imaging lens disclosed in Patent Reference, the secondlens is made of a material having Abbe's number between 23 and 40, andthe third lens is made of a material having Abbe's number between 50 and85. Furthermore, according to the imaging lens, a ratio (f/D) of a focallength f of the whole lens system and a distance D from an incidentsurface on the object side to an image-forming surface is restrainedwithin certain ranges. Accordingly, it is possible to obtain a wideangle of view and a small size, and also satisfactorily correct achromatic aberration.

According to the imaging lens disclosed in Patent Reference, althoughthe number of lenses that compose the imaging lens is as few as four, animaging angle of view is wide and it is also possible to relativelysatisfactorily correct aberrations. However, demands for such awide-angle imaging lens have become diversified each year, andespecially in these years, there are strong demands for being capable ofmanufacturing the imaging lens inexpensively, i.e., an imaging lens thatis easy to assemble with high productivity, as well as the demands to becompatible to high-resolution imaging elements and to have a small size.

In case of the conventional wide-angle imaging lens including theimaging lens disclosed in Patent Reference, the first lens has verystrong negative refractive power relative to other lenses in order toattain a wide angle of view. For this reason, a curvature radius of animage plane-side surface of the first lens is small, and thereby aso-called semispherical ratio is close to 1.0 (semispherical shape),which results in poor workability of the lens.

Further, the image plane-side surface of the first lens is frequentlycoated with an antireflection coating or the like, and there is aserious issue of insufficient coating a periphery of the lens surface incase of a lens having the semispherical ratio near 1.0 described above.Furthermore, in case of the imaging lens, in which the first lens hasstrong refractive power and has the semispherical ratio near 1.0, thesensitivity to deterioration of image-forming performance due todecentering (eccentricity), tilting, etc. occurred upon manufacturing ofthe imaging lens, i.e., production error sensitivity, is high, and thereis a limit by itself for reduction of the manufacturing cost.

Here, those issues are not unique to an imaging lens for mounting on anonboard camera, but are common in imaging lenses for mounting inrelatively small cameras, such as cellular phones, digital stillcameras, portable information terminals, security cameras, networkcameras, video conferencing cameras, fiberscopes, and encapsulatedendoscopes.

In view of the above-described problems of conventional techniques,there is provided an invention, an object of which is to provide animaging lens that has a wide imaging angle of view despite of a smallsize thereof and can suitably reduce the manufacturing cost.

Further objects and advantages of the present invention will be apparentfrom the following description of the present invention.

SUMMARY OF THE INVENTION

In order to attain the objects described above, according to a firstaspect of the invention, an imaging lens includes a first lens that hasnegative refractive power; a second lens having positive refractivepower; a third lens having positive refractive power; a stop; and afourth lens having positive refractive power, arranged in the order froman object side to an image plane side. The first lens has an imageplane-side surface having a positive curvature radius. The second lenshas an image plane-side surface having negative curvature radius. Thethird lens has an image plane-side surface having a negative curvatureradius. The fourth lens has an object-side surface having a positivecurvature radius and an image plane-side surface having a negativecurvature radius. Furthermore, when the whole lens system has a focallength f, the first lens has a focal length f1, and the first lens hasAbbe's number νd1, the imaging lens of the invention having theabove-described configuration satisfies the following conditionalexpressions (1) and (2):−75<f1/f<−5.0  (1)45<νd1<70  (2)

When the imaging lens satisfies the conditional expression (1), it ispossible to restrain a chromatic aberration, a distortion, anastigmatism, and a field curvature within satisfactory rangesrespectively in a balanced manner, while attaining downsizing of theimaging lens. When the value exceeds the upper limit of “−5.0”, thenegative refractive power of the first lens is strong relative to thewhole lens system, which is advantageous for correction of thedistortion, the astigmatism, a chromatic aberration of magnification,etc. However, since an axial chromatic aberration is insufficientlycorrected (a focal position at a short wavelength moves to the objectside relative to a focal position at a reference wavelength), it isdifficult to obtain satisfactory image-forming performance. Furthermore,since incident pupil moves to the object side, a back focal length islong, so that it is difficult to downsize the imaging lens.

On the other hand, when the value is below the lower limit of “−75”, thenegative refractive power of the first lens is weak relative to thewhole lens system, the chromatic aberration of magnification isinsufficiently corrected, and negative distortion increases.Furthermore, since periphery of the image-forming surface curves to theobject side, it is difficult to restrain the field curvature within asatisfactory range. Therefore, also in this case, it is difficult toobtain satisfactory image-forming performance.

In most cases, the first lens of a conventional wide-angle imaging lenshas strong refractive power relative to that of the whole lens system.Also in case of the imaging lens of the invention, the first lens hasnegative refractive power in order to attain a wide angle of view.However, as shown in the conditional expression (1), the first lens hasweak refractive power relative to that of the whole lens system.Therefore, the curvature radius of the image plane-side surface of thefirst lens is large, and so-called hemispheric ratio is away from 1.0.Therefore, an image plane-side concave surface of the first lens has ahalf-elliptic shape that is recessed in a direction perpendicular to theoptical axis. For this reason, according to the imaging lens of theinvention, it is easy to evenly apply coating such as anti-reflectioncoating, and it is possible to improve the yield upon manufacturing theimaging lens. Furthermore, since the first lens has relatively weakrefractive power, it is possible to suitably reduce sensitivity(production error sensitivity) to deterioration of the image-formingperformance due to decentering (eccentricity), tilting, etc. occurredupon manufacturing the imaging lens.

When the imaging lens satisfies the conditional expression (2), it ispossible to effectively restrain occurrence of the chromatic aberration.Having Abbe's number of the first lens greater than the lower limit of“45”, it is possible to effectively restrain the chromatic aberrationgenerated in the first lens. Furthermore, generally in case of awide-angle imaging lens, the first lens has the largest effectivediameter. Having the Abbe's number of the first lens smaller than theupper limit of “75”, it is not necessary to use an expensive material,and it is possible to suitably attain reduction of manufacturing costfor the imaging lens.

The imaging lens having the above-described configuration may bepreferably configured to further satisfy the following conditionalexpression (1-A). When the imaging lens satisfies the conditionalexpression (1-A), it is possible to satisfactorily correct aberrationsand more effectively restrain manufacturing cost by reducingunsatisfactory application of coating such as anti-reflection coatingand reduction of the production error sensitivity.−50<f1/f<−10  (1-A)

According to a second aspect of the present invention, in the imaginglens having the above-described configuration, the first lens may bepreferably formed as an aspheric shape so as to have stronger negativerefractive power as it goes from the optical axis towards the periphery.As described above, according to the first aspect of the presentinvention, the first lens has weaker refractive power than conventionalone. For this reason, how to correct the filed curvature is a key. Fromthis point of view, in case of the imaging lens of the invention, sincethe first lens has strong refractive power at the periphery than thatnear the optical axis, correction at the periphery of the image-formingsurface is satisfactorily made, and aberrations including the fieldcurvature are satisfactorily corrected.

According to a third aspect of the present invention, in the imaginglens having the above-described configuration, the focal length of thefirst lens, the focal length of the second lens, and the focal length ofthe third lens may be preferably longer than three times the focallength of the fourth lens, respectively.

As well known, an imaging element of a CCD sensor, CMOS sensor, or thelike has a predetermined so-called “chief ray angle (CRA)”, a range ofan incident angle of a light beam that can be taken by the sensor.Restraining the incident angle of a light beam emitted from the imaginglens to the image plane within the CRA range, it is possible to suitablyrestrain generation of shading, which is a phenomenon of having darkperiphery in an image. For this reason, with the fourth lens, which isdisposed most closely to the image plane, has the strongest refractivepower, the imaging lens of the invention can have a configurationcapable of suitably restraining the incident angle of a light beamemitted from the imaging lens to the imaging element.

As a result of optical simulations, it was found that it is achievableto obtain a wide angle of view, corrections of aberrations, etc. in abalanced manner while attaining downsizing of the imaging lens, byhaving the focal length of the first lens, the focal length of thesecond lens, and the focal length of the third lens longer than threetimes the focal length of the fourth lens. Furthermore, since three outof the four lenses have weak refractive powers as described above, it ispossible to even more effectively reduce the production errorsensitivity.

In view of reduction of the manufacturing cost, it is preferred to formeach lens from a resin material. However, for example, in case of animaging lens for an onboard camera to be mounted in an automobile, sinceit is not rare that a temperature inside the vehicle exceeds 70° C. inthe midsummer hot sun, it is a critical issue upon designing to restrainfluctuation of the focal length due to the temperature changes. For suchimaging lens used under severe environment, it is conventionallynecessary to form each lens from a glass material, which results inincrease of the manufacturing cost.

Therefore, for the imaging lens having the above-describedconfiguration, it is preferred to form the fourth lens from aglass-based material. As described above, according to the imaging lensof the invention, only the fourth lens has strong refractive power. Forthis reason, forming the fourth lens, which has strong refractive power,from a glass-based material, it is possible to minimize the fluctuationof the focal length of the imaging lens due to temperature changes inthe surrounding environment. On the other hand, three lenses from thefirst to the third lenses have relatively weak refractive power, so thatthe fluctuation of the focal length due to temperature changes is small.Accordingly, it is possible to not only form those three lenses fromglass-based materials, but also form from resin materials.

According to a fourth aspect of the present invention, when the fourthlens has a focal length f4, the imaging lens having the above-describedconfiguration may be preferably configured to satisfy the followingconditional expression (3):1.0<f4/f<2.5  (3)

When the imaging lens satisfies the conditional expression (3), it ispossible to satisfactorily correct distortion while attaining downsizingof the imaging lens. Furthermore, when the imaging lens satisfies theconditional expression (3), it is also possible to suitably restrain anincident angle of a light beam emitted from the imaging lens to animaging element within the CRA range. When the value exceeds the upperlimit of “2.5”, since the fourth lens has weak refractive power,although it is effective for correcting distortion, the axial chromaticaberration is insufficiently corrected, so that it is difficult toobtain satisfactory image-forming performance.

Furthermore, it is difficult to restrain an incident angle of a lightbeam emitted from the imaging lens to the imaging element within the CRArange. On the other hand, when the value is below “1.0”, since thefourth lens has relatively strong refractive power, it is easier torestrain the incident angle of a light beam emitted from the imaginglens to the imaging element within the CRA range. However, since thedistortion increases and the off-axis chromatic aberration ofmagnification is insufficiently corrected, also in this case, it isdifficult to obtain satisfactory image-forming performance.

According to a fifth aspect of the present invention, when the fourthlens has a focal length f4 and a composite focal length of the secondlens and the third lens is f23, the imaging lens having theabove-described configuration may be preferably configured to satisfythe following conditional expression (4):0.2<f4/f23<1.0  (4)

When the imaging lens satisfies the conditional expression (4), it ispossible to restrain an astigmatism, a chromatic aberration, and adistortion within satisfactory ranges in a balanced manner. When thevalue exceeds the upper limit of “1.0”, among the lenses having positiverefractive power, the fourth lens has relative weak refractive power,which is effective for correcting the off-axis chromatic aberration ofmagnification, the axial chromatic aberration is insufficientlycorrected and astigmatic difference increases, so that it is difficultto obtain satisfactory image-forming performance. On the other hand,when the value is below “0.2”, among the lenses having positiverefractive power, the fourth lens has relatively strong refractivepower, and the off-axis chromatic aberration of magnification isinsufficiently corrected and a sagittal image surface of the astigmatismcurves to the object side. Furthermore, since the distortion alsoincreases, it is difficult to obtain satisfactory image-formingperformance.

According to a sixth aspect of the present invention, the when acomposite focal length from the first lens to the third lens is f123,the imaging lens having the above-described configuration may bepreferably configured to satisfy the following conditional expression(5):2.0<f123/f<5.0  (5)

When the imaging lens satisfies the conditional expression (5), it ispossible to satisfactorily correct the chromatic aberration, whileattaining downsizing of the imaging lens. Furthermore, when the imaginglens satisfies the conditional expression (5), it is also possible tosuitably restrain the incident angle of a light beam emitted from theimaging lens to the imaging element within the CRA range. When the valueexceeds the upper limit of “5.0”, since the composite refractive powerof the first lens to the third lens, which are arranged on the objectside relative to the stop, the back focal length is long and it isdifficult to attain downsizing of the imaging lens.

Furthermore, the chromatic aberration of magnification is insufficientlycorrected near periphery of the image and it is difficult to obtainsatisfactory image-forming performance. On the other hand, when thevalue is below “2.0”, although it is effective for downsizing of theimaging lens and satisfactory correction of the chromatic aberration ofmagnification, it is difficult to restrain the incident angle of a lightbeam emitted from the imaging lens to the imaging element within the CRArange.

According to a seventh aspect of the present invention, when a distanceon an optical axis between the first lens and the second lens is dA, theimaging lens having the above-described configuration may be preferablyconfigured to satisfy the following conditional expression (6):0.3<dA/f<1.0  (6)

When the imaging lens satisfies the conditional expression (6), it ispossible to satisfactorily correct the distortion and the astigmatismwhile attaining downsizing of the imaging lens. When the value exceedsthe upper limit of “1.0”, the size of the first lens is big, so that itis difficult to attain downsizing of the imaging lens. Furthermore, anoff-axis chromatic aberration of magnification is insufficientlycorrected and it is difficult to correct periphery of a tangential imagesurface, so that it is difficult to obtain satisfactory image-formingperformance. On the other hand, when the value is below the lower limitof “0.3”, although it is effective for downsizing of the imaging lens, asagittal image surface of the astigmatism curves to the object side, andastigmatic difference increases. Furthermore, since the distortionincreases, it is difficult to obtain satisfactory image-formingperformance.

According to an eighth aspect of the present invention, when the thirdlens has Abbe's number νd3 and the fourth lens has Abbe's number νd4,the imaging lens may be preferably configured to satisfy the conditionalexpressions (7) and (8):20<νd3<40  (7)45<νd4<70  (8)

When the imaging lens satisfies the conditional expressions (7) and (8),it is possible to restrain the chromatic aberration within satisfactoryrange. Forming the third lens and the fourth lens, which are disposedacross the stop from each other, from a material having Abbe's numberwithin the ranges indicated by the conditional expressions (7) and (8),it is possible to satisfactorily correct the axial and off-axischromatic aberrations.

When a half angle of view of the lens system is ω, the imaging lenshaving the above-described configuration preferably satisfies “135°≦2ω”.The imaging lens of the invention is especially effective for an imaginglens that is required to have an angle of view not smaller than 135°.

According to the imaging lens of the invention, it is possible toprovide a small-sized imaging lens that can suitably attain both a wideangle of the imaging lens and reduction of manufacturing cost. Moreover,according to the imaging lens of the invention, it is possible toprovide an imaging lens having less fluctuation in focal length due totemperature changes of the surrounding environment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view of a schematic configuration of an imaginglens in Numerical Data Example 1 according to an embodiment of theinvention;

FIG. 2 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 1;

FIG. 3 is an aberration diagram showing a spherical aberration, anastigmatism, and a distortion of the imaging lens of FIG. 1;

FIG. 4 shows a sectional view of a schematic configuration of an imaginglens in Numerical Data Example 2 according to the embodiment of theinvention;

FIG. 5 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 4;

FIG. 6 is an aberration diagram showing a spherical aberration, anastigmatism, and a distortion of the imaging lens of FIG. 4;

FIG. 7 shows a sectional view of a schematic configuration of an imaginglens in Numerical Data Example 3 according to the embodiment of theinvention;

FIG. 8 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 7;

FIG. 9 is an aberration diagram showing a spherical aberration, anastigmatism, and a distortion of the imaging lens of FIG. 7;

FIG. 10 shows a sectional view of a schematic configuration of animaging lens in Numerical Data Example 4 according to the embodiment ofthe invention;

FIG. 11 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 10;

FIG. 12 is an aberration diagram showing a spherical aberration, anastigmatism, and a distortion of the imaging lens of FIG. 10;

FIG. 13 shows a sectional view of a schematic configuration of animaging lens in Numerical Data Example 5 according to the embodiment ofthe invention;

FIG. 14 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 13;

FIG. 15 is an aberration diagram showing a spherical aberration, anastigmatism, and a distortion of the imaging lens of FIG. 13;

FIG. 16 shows a sectional view of a schematic configuration of animaging lens in Numerical Data Example 6 according to the embodiment ofthe invention;

FIG. 17 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 16; and

FIG. 18 is an aberration diagram showing a spherical aberration, anastigmatism, and a distortion of the imaging lens of FIG. 16.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereunder, referring to the accompanying drawings, an embodiment of thepresent invention will be fully described.

FIGS. 1, 4, 7, 10, 13, and 16 are schematic sectional views of imaginglenses in Numerical Data Examples 1 to 6 according to the embodiment,respectively. Since a basic lens configuration is the same among thoseNumerical Data Examples, the lens configuration of the embodiment willbe described with reference to the illustrative sectional view ofNumerical Data Example 1.

As shown in FIG. 1, the imaging lens of the embodiment includes a firstlens L1 having negative refractive power; a second lens L2 havingpositive refractive power; a third lens L3 having positive refractivepower; an aperture stop ST; and a fourth lens L4 having positiverefractive power, arranged in the order from an object side to an imageplane side. Here, an infrared cutoff filter, a cover glass, or the likemay be provided between the fourth lens L4 and an image plane IM.

The first lens L1 is formed in a shape, such that a curvature radius ofan object-side surface thereof r1 and a curvature radius of an imageplane-side surface thereof r2 are both positive so as to have a shape ofa meniscus lens directing a convex surface thereof to the object sidenear an optical axis X. In addition, the first lens L1 is formed as anaspheric shape so as to have stronger refractive power as it is close tothe periphery from the optical axis X.

More specifically, the first lens L1 has stronger negative refractivepower as it goes to the periphery from a part that is around 70% of themaximum effective diameter. Here, the shape of the first lens L1 is notlimited to the shape of a meniscus lens directing a convex surfacethereof to the object side near the optical axis X. The first lens L1can have any shape as long as the curvature radius of the imageplane-side surface thereof r2 is positive, and can be shaped such thatthe curvature radius r1 is negative, i.e. a shape of biconcave lens nearthe optical axis X. Numerical Data Examples 1 and 2 are examples, inwhich the shape of the first lens L1 is that of a meniscus lensdirecting the convex surface thereof to the object side near the opticalaxis X, and Numerical Data Examples 3 to 6 are examples, in which theshape of the first lens L1 is that of a biconcave lens near the opticalaxis X.

The second lens L2 is formed in a shape such that a curvature radius ofan object-side surface thereof r3 and a curvature radius of an imageplane-side surface thereof r4 are both negative and is formed so as tohave a shape of a meniscus lens directing a concave surface thereof tothe object side near the optical axis X. Among them, the imageplane-side surface of the second lens L2 is formed as an asphericsurface directing a concave surface thereof to the object side near theoptical axis X and directing a convex surface thereof to the object sideat the lens periphery. In short, the second lens L2 of the embodiment isformed as an aspheric surface shape such that an image plane-sidesurface thereof has an inflexion point and has a shape of a meniscuslens directing a concave surface thereof to the object side near theoptical axis X, and has a shape of a biconcave lens at lens peripherythat is away from the optical axis X.

In case of the wide-angle imaging lens such as the one according to theinvention, it is important how to correct the curvature at the peripheryof the image plane for the wide angle of imaging coverage, in order toobtain satisfactory aberration. In this view, with such aspheric shapeof the second lens L2, since the curvature at the periphery of the imageplane is suitably restrained, it is possible to satisfactorily correctthe field curvature. Here, the shape of the second lens L2 is notlimited to that of the meniscus lens directing a concave surface thereofto the object side near the optical axis X. The second lens L2 can haveany shape, as long as the curvature radius of the image plane-sidesurface thereof r4 is negative, and can be formed in a shape such thatthe curvature radius r3 is positive, i.e. a shape of a biconvex lensnear the optical axis X.

The third lens L3 is formed in a shape such that a curvature radius ofan object-side surface r5 is positive and a curvature radius of an imageplane-side surface r6 is negative, so as to have a shape of a biconvexlens near the optical axis X. The shape of the third lens L3 is notlimited to the shape of the biconvex lens near the optical axis X. Thethird lens L3 can have any shape as long as the curvature radius of theimage plane-side surface r6 is negative and can be formed in a shapesuch that the curvature radius r5 is negative, i.e. a shape of ameniscus lens directing a concave surface to the object side near theoptical axis X. Numerical Data Examples 1 to 4 are examples for that thethird lens L3 has a shape of a biconvex lens near the optical axis X,and Numerical Data Examples 5 and 6 are examples for that the third lensL3 has a shape of a meniscus lens directing a concave surface thereof tothe object side near the optical axis X.

According to the imaging lens of the embodiment, the focal length of thefirst lens L1, the focal length of the second lens L2, and the focallength of the third lens L3 are respectively longer than three times thefocal length of the fourth lens L4. In other words, when the first lensL1 has a focal length f1, the second lens L2 has a focal length f2, thethird lens L3 has a focal length f3, and the fourth lens L4 has a focallength f4, the imaging lens of the embodiment satisfies the followingconditional expressions.f1>3×f4, f2>3×f4, and f3>3×f4

The fourth lens L4 is formed in a shape such that a curvature radius ofan object-side surface thereof r8 is positive and a curvature radius ofan image plane-side surface thereof r9 is negative, so as to have ashape of a biconvex lens near the optical axis X.

Furthermore, the imaging lens of the embodiment satisfies the followingrespective conditional expressions. Therefore, according to the imaginglens of the embodiment, it is possible to suitably attain a wide angleof the imaging lens and reduction of the manufacturing cost, as well assatisfactorily correct an aberration in spite of the small size thereof.−75<f1/f<−5.0  (1)45<νd1<70  (2)1.0<f4/f<2.5  (3)0.2<f4/f23<1.0  (4)2.0<f123/f<5.0  (5)0.3<dA/f<1.0  (6)20<νd3<40  (7)45<νd4<70  (8)

In the above conditional expressions:

-   f: Focal length of whole lens system-   f1: Focal length of a first lens L1-   f23: Composite focal length of a second lens L2 and a third lens L3-   f4: Focal length of a fourth lens L4-   f123: Composite focal length from the first lens L1 to the third    lens L3-   dA: Distance on an optical axis between the first lens L1 and the    second lens L2-   νd1: Abbe's number of the first lens L1-   νd3: Abbe's number of the third lens L3-   νd4: Abbe's number of the fourth lens L4

The imaging lens of the embodiment preferably further satisfies thefollowing conditional expression (1-A):−50<f1/f<−10  (1-A)

When the imaging lens satisfies the conditional expression (1-A), thefirst lens L1 has weaker refractive power, so that the first lens L1 isformed in a shape having a small difference between the thickness on anoptical axis and the thickness of periphery, i.e. a shape having a smallchange in the thickness from the lens center to the periphery. With thisconfiguration, it is possible to improve workability of the first lensL1, and also it is easy to evenly apply antireflection coating from thecenter part of the lens to the periphery. Here, Numerical Data Example 1and Numerical Data Examples 3 to 6 are examples that satisfy the aboveconditional expression (1-A), and Numerical Data Example 2 is an examplethat does not satisfy the conditional expression (1-A).

Here, it is not necessary to satisfy all of the respective conditionalexpressions, and it is achievable to obtain an effect corresponding tothe respective conditional expression when any single one of theconditional expressions is individually satisfied.

In the embodiment, any lens surface of the respective lenses is formedas an aspheric surface. When the aspheric surfaces applied to the lenssurfaces have an axis Z in a direction of the optical axis X, a height Hin a direction perpendicular to the optical axis X, a conicalcoefficient k, and aspheric coefficients A₄, A₆, A₈, A₁₀, and A₁₂, ashape of the aspheric surfaces of the lens surfaces is expressed asfollows:

$Z = {\frac{\frac{H^{2}}{R}}{1 + \sqrt{1 - {\left( {k + 1} \right)\frac{H^{2}}{R^{2}}}}} + {A_{4}H^{4}} + {A_{6}H^{6}} + {A_{8}H^{8}} + {A_{10}H^{10}} + {A_{12}H^{12}}}$

Next, Numerical Data Examples of the imaging lens of the embodiment willbe described. In each Numerical Data Example, f represents a focallength of the whole lens system, Fno represents an F number, and 2ωrepresents an angle of view, respectively. In addition, i represents asurface number counted from the object side, r represents a curvatureradius, d represents a distance between lens surfaces (surface spacing)on the optical axis, nd represents a refractive index for a d line, andνd represents Abbe's number for the d line, respectively. Here, asphericsurfaces are indicated with surface numbers i affixed with * (asterisk).In addition, total surface spacing from the object-side surface of thefirst lens L1 to the image plane IM is indicated as L14.

Numerical Data Example 1

Basic data are shown below.

f = 3.13 mm, Fno = 2.8, 2ω = 161.4° Unit: mm Surface Data Surface Numberi r d nd νd (Object) ∞ ∞ 1* 77.022 0.900 1.544 55.5 (=νd1) 2* 36.3162.150 (=dA) 3* −13.337 1.000 1.544 55.5 (=νd2) 4* −7.152 0.090 5* 14.6851.000 1.636 23.9 (=νd3) 6* −37.790 0.350 7 (Stop) ∞ 0.500 8* 3.751 0.8501.544 55.5 (=νd4) 9* −4.549 2.480 (Image ∞ plane) f1 = −127.35 mm f2 =26.83 mm f3 = 16.77 mm f4 = 3.92 mm f123 = 10.86 mm f23 = 10.21 mm L14 =9.32 mm Aspheric Surface Data First Surface k = 0.000, A₄ = −3.528E−04,A₆ = 2.507E−06, A₈ = −3.053E−08, A₁₀ = −1.320E−09 Second Surface k =0.000, A₄ = 5.781E−03, A₆ = −9.955E−05, A₈ = 1.217E−05, A₁₀ = 8.349E−06,A₁₂ = 1.168E−09 Third Surface k = 0.000, A₄ = 7.208E−04, A₆ = −6.424E−04Fourth Surface k = 0.000, A₄ = 9.683E−03, A₆ = 8.629E−03 Fifth Surface k= 0.000, A₄ = 1.927E−02, A₆ = 1.505E−02 Sixth Surface k = −4.226E+01, A₄= 1.860E−02, A₆ = 5.481E−03 Eighth Surface k = 0.000, A₄ = −8.159E−03,A₆ = 6.618E−03 Ninth Surface k = 0.000, A₄ = −2.319E−02, A₆ = 1.532E−02The values of the respective conditional expressions are as follows:f4/f = 1.26 f1/f = −40.75 f4/f23 = 0.38 dA/f = 0.69 f123/f = 3.47

Accordingly, the imaging lens of Numerical Data Example 1 satisfies theabove-described conditional expressions.

FIG. 2 shows a lateral aberration that corresponds to a ratio H of eachimage height to the maximum image height (hereinafter referred to as“image height ratio H”), which is divided into a tangential directionand a sagittal direction (which is the same in FIGS. 5, 8, 11, 14, and17). Furthermore, FIG. 3 shows a spherical aberration (mm), anastigmatism (mm), and a distortion (%) of the imaging lens in NumericalData Example 1, respectively. In the aberration diagrams, for thelateral aberration diagrams and spherical aberration diagrams,aberrations at each wavelength, i.e. a g line (435.84 nm), an e line(546.07 nm), and a C line (656.27 nm) are indicated. In astigmatismdiagram, an aberration on a sagittal image surface S and an aberrationon a tangential image surface T are respectively indicated (which arethe same in FIGS. 6, 9, 12, 15, and 18). As shown in FIGS. 2 and 3,according to the imaging lens of Numerical Data Example 1, theaberrations are satisfactorily corrected.

Numerical Data Example 2

Basic data are shown below.

f = 3.00 mm, Fno = 2.8, 2ω = 141.0° Unit: mm Surface Data Surface Numberi r d nd νd (Object) ∞ ∞ 1* 30.000 0.893 1.544 55.5 (=νd1) 2* 6.3862.608 (=dA) 3* −3.553 0.704 1.636 23.9 (=νd2) 4* −2.919 0.097 5* 16.8221.314 1.636 23.9 (=νd3) 6* −34.079 0.750 7 (Stop) ∞ 0.630 8* 5.272 0.9981.544 55.5 (=νd4) 9* −3.897 3.021 (Image ∞ plane) f1 = −15.12 mm f2 =17.98 mm f3 = 17.90 mm f4 = 4.29 mm f123 = 13.14 mm f23 = 8.72 mm L14 =11.02 mm Aspheric Surface Data First Surface k = 0.000, A₄ = −8.666E−05,A₆ = 1.504E−05, A₈ = 1.574E−07, A₁₀ = −8.510E−10 Second Surface k =0.000, A₄ = 4.166E−03, A₆ = 2.556E−03, A₈ = −4.986E−04, A₁₀ = 8.632E−05,A₁₂ = 6.831E−10 Third Surface k = 0.000, A₄ = −2.552E−03, A₆ = 1.391E−04Fourth Surface k = 0.000, A₄ = 1.052E−02, A₆ = 3.650E−03 Fifth Surface k= 0.000, A₄ = 1.341E−02, A₆ = 1.219E−02 Sixth Surface k = −4.226E+01, A₄= −5.766E−03, A₆ = 2.322E−02 Eighth Surface k = 0.000, A₄ = −2.203E−02,A₆ = 1.284E−03 Ninth Surface k = 0.000, A₄ = −2.123E−02, A₆ = −3.270E−04The values of the respective conditional expressions are as follows:f4/f = 1.43 f1/f = −5.04 f4/f23 = 0.49 dA/f = 0.87 f123/f = 4.38

Accordingly, the imaging lens of Numerical Data Example 2 satisfies theabove-described conditional expressions.

FIG. 5 shows the lateral aberration that corresponds to the image heightratio H of the imaging lens in Numerical Data Example 2, and FIG. 6shows a spherical aberration (mm), astigmatism (mm), and a distortion(%), respectively. As shown in FIGS. 5 and 6, according to the imaginglens of Numerical Data Example 2, the aberrations are alsosatisfactorily corrected.

Numerical Data Example 3

Basic data are shown below.

f = 3.02 mm, Fno = 2.8, 2ω = 135.0° Unit: mm Surface Data Surface Numberi r d nd νd (Object) ∞ ∞ 1* −248.850 0.800 1.544 55.5 (=νd1) 2* 25.0002.848 (=dA) 3* −3.930 1.158 1.636 23.9 (=νd2) 4* −3.239 0.097 5* 18.2671.057 1.536 23.9 (=νd3) 6* −33.373 0.419 7 (Stop) ∞ 0.630 8* 6.930 0.9981.544 55.5 (=νd4) 9* −3.312 2.839 (Image ∞ plane) f1 = −41.73 mm f2 =17.56 mm f3 = 22.20 mm f4 = 4.27 mm f123 = 10.63 mm f23 = 9.41 mm L14 =10.84 mm Aspheric Surface Data First Surface k = 0.000, A₄ = −1.289E−04,A₆ = 1.462E−05, A₈ = −1.105E−07, A₁₀ = −8.510E−10 Second Surface k =0.000, A₄ = 3.253E−03, A₆ = −6.480E−05, A₈ = −6.984E−06, A₁₀ =2.641E−06, A₁₂ = 6.831E−10 Third Surface k = 0.000, A₄ = 1.680E−03, A₆ =1.677E−03 Fourth Surface k = 0.000, A₄ = 1.600E−02, A₆ = 5.051E−03 FifthSurface k = 0.000, A₄ = 6.517E−03, A₆ = 1.653E−02 Sixth Surface k =−4.226E+01, A₄ = −1.276E−02, A₆ = 3.172E−02 Eighth Surface k = 0.000, A₄= 2.268E−03, A₆ = 3.209E−04 Ninth Surface k = 0.000, A₄ = −2.291E−03, A₆= 3.967E−03 The values of the respective conditional expressions are asfollows: f4/f = 1.41 f1/f = −13.82 f4/f23 = 0.45 dA/f = 0.94 f123/f =3.52

Accordingly, the imaging lens of Numerical Data Example 3 satisfies theabove-described conditional expressions.

FIG. 8 shows the lateral aberration that corresponds to the image heightratio H of the imaging lens in Numerical Data Example 3, and FIG. 9shows a spherical aberration (mm), an astigmatism (mm), and a distortion(%), respectively. As shown in FIGS. 8 and 9, according to the imaginglens of Numerical Data Example 3, the aberrations are alsosatisfactorily corrected.

Numerical Data Example 4

Basic data are shown below.

f = 3.30 mm, Fno = 2.8, 2ω = 168.0° Unit: mm Surface Data Surface Numberi r d nd νd (Object) ∞ ∞ 1* −248.850 0.890 1.544 55.5 (=νd1) 2* 96.2732.800 (=dA) 3* −10.597 1.150 1.636 23.9 (=νd2) 4* −6.482 0.090 5* 24.2051.050 1.636 23.9 (=νd3) 6* −21.063 0.400 7 (Stop) ∞ 0.600 8* 5.528 1.0001.544 55.5 (=νd4) 9* −3.791 2.678 (Image ∞ plane) f1 = −127.54 mm f2 =23.69 mm f3 = 17.88 mm f4 = 4.30 mm f123 = 10.53 mm f23 = 10.06 mm L14 =10.66 mm Aspheric Surface Data First Surface k = 0.000, A₄ = −2.979E−04,A₆ = 1.105E−05, A₈ = 9.195E−08, A₁₀ = −8.510E−10 Second Surface k =0.000, A₄ = 4.338E−03, A₆ = −6.372E−05, A₈ = −3.828E−06, A₁₀ =2.961E−06, A₁₂ = 6.831E−10 Third Surface k = 0.000, A₄ = 8.486E−04, A₆ =−5.634E−04 Fourth Surface k = 0.000, A₄ = 9.508E−03, A₆ = 7.337E−03Fifth Surface k = 0.000, A₄ = 1.738E−02, A₆ = 1.307E−02 Sixth Surface k= −4.226E+01, A₄ = 1.037E−02, A₆ = 2.811E−03 Eighth Surface k = 0.000,A₄ = −2.094E−02, A₆ = 3.381E−03 Ninth Surface k = 0.000, A₄ =−2.488E−02, A₆ = 2.038E−03 The values of the respective conditionalexpressions are as follows: f4/f = 1.30 f1/f = −38.66 f4/f23 = 0.43 dA/f= 0.85 f123/f = 3.19

Accordingly, the imaging lens of Numerical Data Example 4 satisfies theabove-described conditional expressions.

FIG. 11 shows the lateral aberration that corresponds to the imageheight ratio H of the imaging lens in Numerical Data Example 4, and FIG.12 shows a spherical aberration (mm), astigmatism (mm), and a distortion(%), respectively. As shown in FIGS. 11 and 12, according to the imaginglens of Numerical Data Example 4, the aberrations are alsosatisfactorily corrected.

Numerical Data Example 5

Basic data are shown below.

f = 3.30 mm, Fno = 2.8, 2ω = 146.7° Unit: mm Surface Data Surface Numberi r d nd νd (Object) ∞ ∞ 1* −248.850 0.893 1.544 55.5 (=νd1) 2* 115.3672.814 (=dA) 3* −16.119 1.155 1.636 23.9 (=νd2) 4* −8.116 0.097 5*−63.307 1.050 1.636 23.9 (=νd3) 6* −9.562 0.420 7 (Stop) ∞ 0.630 8*7.276 0.998 1.544 55.5 (=νd4) 9* −3.238 2.741 (Image ∞ plane) f1 =−144.83 mm f2 = 24.36 mm f3 = 17.59 mm f4 = 4.26 mm f123 = 10.68 mm f23= 10.26 mm L14 = 10.80 mm Aspheric Surface Data First Surface k = 0.000,A₄ = −2.576E−04, A₆ = 1.094E−05, A₈ = −6.006E−08, A₁₀ = −8.510E−10Second Surface k = 0.000, A₄ = 4.090E−03, A₆ = −6.675E−05, A₈ =−8.552E−07, A₁₀ = 2.301E−06, A₁₂ = 6.831E−10 Third Surface k = 0.000, A₄= 1.670E−03, A₆ = −2.059E−04 Fourth Surface k = 0.000, A₄ = 8.602E−03,A₆ = 8.250E−03 Fifth Surface k = 0.000, A₄ = 1.921E−02, A₆ = 1.172E−02Sixth Surface k = −4.226E+01, A₄ = 1.169E−02, A₆ = 2.427E−03 EighthSurface k = 0.000, A₄ = −1.509E−02, A₆ = 4.174E−03 Ninth Surface k =0.000, A₄ = −2.029E−02, A₆ = 3.808E−03 The values of the respectiveconditional expressions are as follows: f4/f = 1.29 f1/f = −43.89 f4/f23= 0.42 dA/f = 0.85 f123/f = 3.24

Accordingly, the imaging lens of Numerical Data Example 5 satisfies theabove-described conditional expressions.

FIG. 14 shows the lateral aberration that corresponds to the imageheight ratio H of the imaging lens in Numerical Data Example 5, and FIG.15 shows a spherical aberration (mm), an astigmatism (mm), and adistortion (%), respectively. As shown in FIGS. 14 and 15, according tothe imaging lens of Numerical Data Example 5, the aberrations are alsosatisfactorily corrected.

Next, the imaging lens of Numerical Data Example 6 will be described.According to the imaging lens of Numerical Data Example 6, the fourthlens L4 is made of a glass-based material. Therefore, according to theimaging lens of Numerical Data Example 6, it is possible to suitablyrestrain fluctuation of the focal length due to temperature changes inthe surrounding environment.

Numerical Data Example 6

Basic data are shown below.

f = 3.29 mm, Fno = 2.8, 2ω = 146.0° Unit: mm Surface Data Surface Numberi r d nd νd (Object) ∞ ∞ 1* −248.850 0.893 1.544 55.5 (=νd1) 2* 122.0721.500 (=dA) 3* −23.343 1.000 1.636 23.9 (=νd2) 4* −7.926 0.097 5*−149.695 1.000 1.636 23.9 (=νd3) 6* −7.011 0.200 7 (Stop) ∞ 0.800 8*8.308 0.998 1.619 63.9 (=νd4) 9* −4.392 2.445 (Image ∞ plane) f1 =−150.48 mm f2 = 18.42 mm f3 = 11.54 mm f4 = 4.79 mm f123 = 7.41 mm f23 =7.20 mm L14 = 8.93 mm Aspheric Surface Data First Surface k = 0.000, A₄= −2.576E−04, A₆ = 1.094E−05, A₈ = −6.006E−08, A₁₀ = −8.510E−10 SecondSurface k = 0.000, A₄ = 1.196E−02, A₆ = −3.436E−04, A₈ = 1.043E−05, A₁₀= 4.409E−05, A₁₂ = 6.831E−10 Third Surface k = 0.000, A₄ = 1.068E−03, A₆= −8.122E−04 Fourth Surface k = 0.000, A₄ = 9.022E−03, A₆ = 9.202E−03Fifth Surface k = 0.000, A₄ = 2.915E−02, A₆ = 1.310E−02 Sixth Surface k= −4.226E+01, A₄ = −1.975E−03, A₆ = 1.276E−02 Eighth Surface k = 0.000,A₄ = −1.941E−02, A₆ = 3.219E−03 Ninth Surface k = 0.000, A₄ =−3.233E−02, A₆ = 2.626E−03 The values of the respective conditionalexpressions are as follows: f4/f = 1.46 f1/f = −45.71 f4/f23 = 0.67 dA/f= 0.46 f123/f = 2.25

Accordingly, the imaging lens of Numerical Data Example 6 satisfies theabove-described conditional expressions. Therefore, according to theimaging lens, it is possible to satisfactorily correct aberrations inspite of the wide angle thereof.

FIG. 17 shows a lateral aberration that corresponds to the image heightratio H of the imaging lens in Numerical Data Example 6, and FIG. 18shows a spherical aberration (mm), an astigmatism (mm), and a distortion(%), respectively. As shown in FIGS. 17 and 18, according to the imaginglens of Numerical Data Example 6, the aberrations are alsosatisfactorily corrected.

According to the imaging lens of the embodiment described above, it isachievable to obtain an angle of view (2ω) that is not smaller than135°. For reference, the imaging lenses of Numerical Data Example 1 to 6attain the angles of view that are as wide as 135.0° to 168.0°.

Here, according to each Numerical Data Example, a surface of each lensis formed as an aspheric surface, but if there is certain flexibility inthe total length of the imaging lens or required optical performances,it is also possible to form all or a part of the lens surfaces asspherical surfaces.

Accordingly, when the imaging lens of the embodiment is applied in anoptical system for mounting in cameras such as cellular phones, digitalstill cameras, portable information terminals, security cameras, onboardcameras, network cameras, video conferencing cameras, fiberscopes, andcapsulated endoscopes, it is achievable to obtain both highfunctionality and downsizing of the cameras.

The invention may be applied in an imaging lens for mounting in a devicethat is required to have a wide angle of view as an imaging lens andsatisfactory aberration correction performance, as well as to minimizefluctuation of a focal length due to temperature changes in thesurrounding environment or other cause, e.g. a security camera oronboard camera. Furthermore, the invention may be applied in an imaginglens for mounting in a device, which is required to have a wide angle ofview as an imaging lens and to be inexpensive, for example a camera suchas a cellular phone, smartphone, network camera, and encapsulatedendoscope.

The disclosure of Japanese Patent Application No. 2012-262095, filed onNov. 30, 2012, is incorporated in the application by reference.

While the present invention has been explained with reference to thespecific embodiments of the present invention, the explanation isillustrative and the present invention is limited only by the appendedclaims.

What is claimed is:
 1. An imaging lens comprising: a first lens having negative refractive power; a second lens having positive refractive power; a third lens having positive refractive power; a stop; and a fourth lens having positive refractive power, arranged in this order from an object side to an image plane side, wherein said first lens is formed in a shape so that a surface thereof on the image plane side have a positive curvature radius, said second lens is formed in a shape so that a surface thereof on the image plane side have a negative curvature radius, said third lens is formed in a shape so that a surface thereof on the image plane side has a negative curvature radius, said fourth lens is formed in a shape so that a surface thereof on the object side has a positive curvature radius and a surface thereof on the image plane side has a negative curvature radius, and said first lens has a focal length f1 and an Abbe's number νd1 so that the following conditional expressions are satisfied: −75<f1/f<−5.0 45<νd1<70 where f is a focal length of a whole lens system.
 2. The imaging lens according to claim 1, wherein said first lens is formed in an aspheric shape so as the first lens has refractive power increasing from an optical axis thereof toward a periphery thereof.
 3. The imaging lens according to claim 1, wherein each of said first lens, said second lens, and said third lens has a focal length three times greater than that of the fourth lens.
 4. The imaging lens according to claim 1, wherein said fourth lens has a focal length f4 so that the following conditional expression is satisfied: 1.0<f4/f<2.5 where f is the focal length of the whole lens system.
 5. The imaging lens according to claim 1, wherein said second lens and said third lens have a composite focal length f23, and said fourth lens has a focal length f4 so that the following conditional expression is satisfied: 0.2<f4/f23<1.0.
 6. The imaging lens according to claim 1, wherein said first lens, said second lens, and said third lens have a composite focal length f123 so that the following conditional expression is satisfied: 2.0<f123/f<5.0 where f is the focal length of the whole lens system.
 7. The imaging lens according to claim 1, wherein said first lens is situated away from the second lens by a distance dA on an optical axis thereof so that the following conditional expression is satisfied: 0.3<dA/f<1.0 where f is the focal length of the whole lens system.
 8. The imaging lens according to claim 1, wherein said third lens has an Abbe's number νd3 and said fourth lens has Abbe's number νd4 so that the following conditional expressions are satisfied: 20<νd3<40 45<νd4<70 